It is known to form an omnidirectional antenna by stacking a plurality of biconical antennas on top of each other. Each biconical antenna is formed by a pair of truncated flared-apart reflecting surfaces. The truncated flared apart surfaces are often conically shaped, with the convex sides of the conical sheets facing one another.
When stacking antennas on top of each other, such as when the antennas are positioned on the mast of a ship or submarine, an antenna feed cable for one biconical antenna must be routed past all antennas located below that antenna. Such cable routings can be critical, because a misplaced cable can alter the radiation pattern of any antenna it passes.
This presents a problem when a wideband biconical antenna design is used beneath an antenna positioned above it within a vertical, cylindrical volume, such as a cylindrical radome, with is the case for submarine communications masts. The challenge is to create a feed that attaches to the upper antenna and that traverses the space occupied by the wideband biconical antenna below. The feed for the antenna positioned above the wideband biconical antenna must not significantly impact the performance of the wideband biconical antenna. In addition, the feeds for each antenna must also be able to reach the hardware that exists at the bottom of the cylindrical volume (in the case of a submarine communications mast).
In some cases, feed cables (e.g., coaxial cables) for the biconical antennas are routed down the center of the stack to avoid complicating the antenna patterns. While providing mechanical stability of the antenna cones (via a central metal tube), such arrangements can limit the antenna's bandwidth. In other arrangements, the feed cables are positioned outside the bicones, for each antenna above another on the vertical stack. This arrangement of the feed cables has the advantage of eliminating the need for routing the cables through the center of the antenna elements in each array. It can, however, induce interference with the outgoing signal, can distort the omnidirectional radiation pattern, and can induce interference with the incoming signal.
Thus, there is a need for an improved feed design for a biconical antenna that enables feed cables to be routed past lower antenna elements to upper antenna elements without affecting the characteristics of the biconical antenna. Specifically, the feed design should not degrade the wideband biconical antenna's input impedance or gain radiation patterns